Vascular prostheses, commonly referred to as “stents,” are now widely used in interventional procedures for treating lesions of the coronary arteries and other vessels. Such devices generally have a tubular shape and are deployed in a vessel to restore and maintain the patency of a segment of a vessel. More recently, such vascular prostheses have been used in combination with local drug delivery and/or radiation therapy to prevent restenosis of a vessel.
Previously-known vascular prostheses are generally either self-expanding or plastically deformable, and such stents have been used outside the cardiac vasculature with mixed success. Whereas stenting is most commonly performed to treat narrowing of the cardiac vessels, more recent efforts have focused on the use of such devices to treat occlusive diseases of the carotid arteries, renal arteries and superficial femoral arteries. Stents used for such applications frequently require a different set of structural characteristics than those typically used in cardiac stenting.
U.S. Pat. No. 4,733,665 to Palmaz is typical of plastically deformable stents, which are delivered transvascularly via a balloon catheter. The stents described in that patent consist of a wire mesh tube or slotted metal tube. The stents are crimped around the balloon of a delivery catheter, and deployed by inflating the balloon at high pressure to plastically deform and expand the struts of the stent. Although such stents have proved adequate for treating occlusive disease of the cardiac vessels, they are subject to a number of well-documented drawbacks when used outside the cardiac vasculature.
For example, previously known plastically deformable stents generally are not appropriate for blood vessels that are subject to compressive or other forms of dynamic loading, such as the arteries in the extremities or the carotid arteries. While they generally provide adequate radial strength, they typically also have a high degree of axial rigidity. Thus, plastically deformable stents should not be employed in vessels that routinely experience longitudinal shape changes, because the stents lack flexibility to conform to the vessel, and may fracture, deform or cause dissection of the vessel.
Much effort has been expended in the last decade on designing flexible axial links that permit adjacent circumferential rings of a plastically deformable stent to bend and conform to the shape of a vessel during delivery, e.g., as described in U.S. Pat. No. 5,195,984 to Schatz. Such links, however, also comprise a plastically deformable material. Although the links are capable of undergoing a limited amount of bending strain during initial deployment, they will quickly work-harden and fracture when subjected to multiple bending cycles, e.g., in a peripheral vessel that is subject to bending.
Additionally, because plastically deformable stents have very little resilience, the stents of the foregoing patents are not suitable for use in vessels that are subject to high radially compressive forces, such as the carotid arteries. Because the carotid arteries lie relatively close to the surface of the neck, there is a substantial risk that the stent may be inadvertently crushed by a blow or other pressure to the neck. For this reason, self-expanding stents, such as the mesh-tube structures described in U.S. Pat. No. 4,655,771 to Wallsten, and tubes formed of superelastic shape memory materials have been the primary focus for vessels subject to dynamic loading.
Self-expanding stents generally are formed as wire mesh tubes, such as in the above-described patent to Wallsten, tubes comprising single or multiple circumferential rings, such as described in U.S. Pat. No. 4,580,568 to Gianturco, coiled sheets, as described in U.S. Pat. No. 4,740,207 to Kreamer, or self-expanding helixes, as described in U.S. Pat. No. 4,665,918 to Garza et al.
Self-expanding wire mesh tubes of the type described in the above patent to Wallsten, and coiled sheet tubes as described in the above patent to Kreamer, provide a high degree of crush resistance, but only limited capability to flex longitudinally or sustain axial compressive loads. Self-expanding ring structures, such as described in the above patent to Gianturco, also provide good crush radial crush resistance, but do not provide high radial strength, and are subject to migration if subjected to cyclic compression.
Perhaps most promising for such applications, helical stents of the type described in the foregoing patent to Garza appear capable of withstanding longitudinal flexure and radial compressive loads. However, even self-expanding helical stents are not expected to perform adequately when subjected to cyclic axially compressive and/or torsional loading, such as encountered in the superficial femoral arteries (“SFA”).
The femoral arteries extend from the iliac arteries in the groin region towards the lower extremities, with the SFAs supplying blood to the knees and feet. Patients suffering from diseases that occlude these vessels, such as arteriosclerosis and vascular complications of diabetes, often may suffer reduced mobility and in extreme cases, may require amputation.
During flexure of the thigh, the femoral artery is subjected to axial compression and/or torsion, which are expected to cause a self-expanding helical stent to undergo radial compression. When such compression is accompanied by shortening of the vessel, the stent is likely to migrate away from its delivery site once the compressive load is removed and the vessel radially re-expands. Although the elastic behavior of the stent is desirable and permits the stent to cope with radial compressive loads, this same feature exacerbates the potential for stent migration when radial compression is accompanied by changes in the vessel length. Consequently, previously known self-expanding helical stents are not expected to perform satisfactorily when deployed in the SFAs and other vessels that experience cyclic axial and/or torsional loading.
In addition to plastically deformable and self-expanding structures, a new type of expandable tubular structure based upon the concept of a “bistable cell” is described in commonly assigned U.S. Patent Application Publication No. US2004/0193247 to Besselink, which is a publication of U.S. application Ser. No. 10/782,266, and which application is incorporated herein by reference in its entirety. As described in that published application, a bistable cell comprises a thick strut joined at its ends to a thin strut so that the thin strut snaps between a stable collapsed and a stable expanded position when subjected to a radially outwardly directed force, but is unstable at any intermediate position.
Although FIG. 10 of the foregoing Besselink application describes the use of flexible links to improve axial flexibility of the stent, as in the above patent to Schatz, that bistable tubular structure would be expected to suffer similar drawbacks to plastically deformable stents when subjected to dynamic axial bending or compressive loads.
In view of the foregoing drawbacks of previously known vascular prostheses, it would be desirable to provide a vascular prosthesis that may be used in blood vessels subject to axial and torsional loading, but which is not prone to migration.
It further would be desirable to provide a vascular prosthesis capable of withstanding high compressive loads without experiencing significant radial strains, thereby avoiding the potential for the axial migration when the compressive loads are accompanied by vessel length changes.
It also would be desirable to provide a vascular prosthesis having high radial strength, but which also is capable of bending along its length with a high degree of resistance to bending fatigue once deployed in a body vessel.
It still further would be desirable to provide a vascular prosthesis having high radial strength, so as to maintain contact with a vessel wall in the presence of compressive loads, but which also is resistant to failure due to cyclically applied axial compressive and tensile loads.
In addition to the lack of a suitable prosthesis design for stenting the SFAs and similar vessels, interventional procedures involving such arteries pose other difficulties, particularly with respect to sizing and placement of stents in those vessels. Generally, it is necessary to ascertain the size, shape, and location of a treatment site, prior to stent placement, to ensure the proper selection of the stent to be used in a particular patient. A number of technologies have been developed to obtain such information, including fluoroscopic visualization using contrast agents, magnetic resonance imaging and computer-assisted tomography. While the latter two methods provide excellent views of the vessel topography, this information typically is static, pre-procedure information, rather than real-time.
In previously known methods for real-time visualization of a target site, a contrast agent is injected into the vessel prior to stent placement to gain information about a treatment site. Often, the use of contrast agents provides less than ideal precision, for example, because the contrast agent tends to disperse once introduced into the bloodstream. This in turn may require the use of larger volumes of contrast agent. In addition, where the vessel is highly occluded, there may be very little flow and thus difficult to assess the size and severity of the occlusion.
Consequently, it would be desirable to provide methods and apparatus for placing a vascular prosthesis within a vessel that reduces the volume of contrast agent required to obtain information about the treatment site.